When you are standing or kneeling and aiming down sights (toggled) you cannot go prone directly, but first have to explicitly toggle out of ADS.
This happens, when the ALT key is mapped to the action "prone". Maybe other keys are affected, too.

This is very annoying and tends to get you killed often, because you are too slow hitting the dirt.
It is also a kind of forced micromanagement of stance and ADS which breaks fluid play and immersion.

Please simply allow going prone while in ADS.
If that is not possible or desired by desigm, then at least toggle ADS automatically, when the player presses the "prone" button.